The rate of carbon consumption is an important factor in a number of metallurgical processes. For example, in the production of anhydrous aluminum chloride, from aluminous raw material, as a precursor for producing aluminum, carbon or carbon monoxide is used as a reductant according to the following equations: EQU Al.sub.2 O.sub.3 +3/2C+3Cl.sub.2 .fwdarw.2AlCl.sub.3 +3/2CO.sub.2 ( 1) EQU Al.sub.2 O.sub.3 +3CO+3Cl.sub.2 .fwdarw.2AlCl.sub.3 +3CO.sub.2 ( 2)
The rate of reaction (2) is significantly higher than reaction (1). However, the use of carbon monoxide as a reductant in reaction (2) requires the generation of carbon monoxide from carbon. Thus, for either chlorination reaction, the activation of carbon is desirable to both increase the rate of chlorination and/or lower the reaction temperature. Carbon monoxide as a reductant results in a rapid chlorination rate and produces an AlCl.sub.3 product with essentially no environmental contaminants such as chlorinated hydrocarbons including polychlorinated biphenyls (PCB's) such as decachlorobiphenyl (DCB) or hexachlorobenzene (HCB), pentachlorobenzonitrile (PCBN), pentachloropyridine (PCP) and octachlorostyrene (OCS). Its cost, however is site dependent. Moreover, use of CO requires a C/Al.sub.2 O.sub.3 mole ratio of 3 which translates to about 0.67 lb C/lb Al and requires a significant gas volume to be handled resulting in higher capital cost. Solid reductants can result in stable cost regardless of site location and can result in lower off-gas volume depending on chlorination temperature.
Petroleum coke is a known source for solid carbon reductants for the chlorination of aluminous materials, such as partially calcined alumina (PCA), metal grade alumina (MGA) and partially calcined aluminum chloride hexahydrate (ACH). Green petroleum coke, i.e., uncalcined coke, is known to have a moderate level of activity. However, a serious disadvantage is that it contains significant quantities of hydrogen and hydrocarbons which are chlorinated during the chlorination process. Not only does their presence result in increased consumption of expensive unrecoverable chlorine, but the resulting chlorinated hydrocarbons contaminate the valuable product.
Petroleum coke then is typically fully calcined at temperatures of 1200.degree.-1400.degree. C. to remove moisture and drive off the hydrogen and hydrocarbons. However, the high temperature calcination of coke, sometimes called dead burning, produces coke with a low surface area and low activity as far as chlorination reaction kinetics are concerned.
An alternative to both carbon monoxide and fully calcined coke is the use of partially calcined coke. U.S. Pat. No. 4,284,607 to Culleiton teaches a method of producing partially calcined coke, involving calcining green petroleum coke in a nitrogen or non-oxidizing atmosphere to a partial calcination temperature of from 650.degree. C.-900.degree. C. U.S. Pat. No. 3,937,786 to Nemecz also discloses producing aluminum chlorides by calcining a mixture of an aluminous material and a carbon-containing reducing agent. The reducing agent specifically disclosed is a high ash containing coal which has been calcined or coked in a reducing atmosphere such as nitrogen. Both references exemplify known methods wherein the calcination of the hydrocarbon-containing carbon reductant, alone or in conjunction with the aluminous materials to be chlorinated, is in a reducing atmosphere without regard to the presence of precursors for harmful chlorinated hydrocarbons. The partially calcined coke is a relatively active reductant resulting in rapid chlorination of aluminous materials such as metal grade alumina to produce anhydrous aluminum chloride. However, the serious disadvantage of such partially calcined coke when used as a reductant for chlorination reactions is that it contains relatively high levels of residual hydrocarbons. During chlorination, utilizing such partially calcined coke as the reductant, the residual hydrocarbons will also be chlorinated to produce compounds such as polychlorinated biphenyls (PCB's), hexachlorobenzene (HCB), decachlorobiphenyl (DCB), pentachlorobenzonitrile (PCBN), pentachloropyridine (PCP), and octochlorostyrene (OSC). In addition, U.S. Pat. No. 4,284,607 and 3,937,786 are directed to methods wherein the reductant for chlorination is produced in-situ with the aluminous feed material to be chlorinated.
Similarly, U.S. Pat. No. 4,073,872 to Willhoft discloses a process for producing an aluminum product by chlorination procedures involving a carbonation step of heating an intimate mixture of aluminum-containing materials and a solid carbonizable organic material at a temperature of 500.degree.-1000.degree. C. After carbonation, the mixture of aluminous material and carbon distributed therein is subjected to chlorination. Although carbonization in either an oxidizing or reducing atmosphere is taught, this reference similarly does not recognize the problems associated with the presence of chlorinated hydrocarbon precursors and, in fact, teaches addition of such precursors as an alternative to pure chlorine during chlorination.
U.S. Pat. No. 4,459,274 to Loutfy et al. discloses a process for preparing a carbon reductant by partial calcination of coke in an oxidizing atmosphere at a temperature of from 650.degree.-950.degree. C. in order to minimize the chlorinated hydrocarbon precursors present during use of the reductant in chlorination processes. When combined with chlorination processes for aluminum-containing materials and other metal oxides, formation of chlorinated hydrocarbons is minimized.
One disadvantage to carbonaceous reductant produced according to U.S. Pat. No. 4,459,274 is that during continuous operation with a given amount of the reductant, the level of chlorinated hydrocarbons can begin to increase prior to full utilization of the reductant present. A second disadvantage is the relatively moderate surface area obtained due to the partial calcination of the coke. This low surface area limits the removal or precursor hydrocarbon from the core of the coke particles. It would be desirable, therefore, to use a calcination process which produces a high surface area. Not only would high surface area enhance the diffusion and removal of precursor hydrocarbon from the core of coke particles, but the efficiency of the chlorination reaction will also generally increase with higher reductant surface area.
A third disadvantage of the process taught by U.S. Pat. No. 4,459,274 is that it is applicable only to calcinations effected under oxidizing conditions and not applicable to those effected under reducing conditions. Calcination under a strong oxidizing atmosphere, such as air and/or oxygen, results in burning off the coke particles from the outside in, i.e. burning the outer surface, and reduces the particle size. Reducing conditions, on the other hand, where such burn does not occur present the opportunity for an increased porosity without change, i.e. reduction, in particle size.
Accordingly it will be desirable to have a solid carbon reductant with a high surface area for high yield during chlorination, that at the same time would be capable of minimizing chlorinated hydrocarbon production over continuous use. It would also be desirable to have a process for producing such a reductant with a reduced level of chlorinated hydrocarbon precursors by calcination of coke containing volatile hydrogen and hydrocarbons in either an oxidizing or reducing atmosphere.